Coolant Storage Tank

ABSTRACT

A coolant storage tank ( 1 ) for storing coolant in a fuel cell system ( 2 ), the coolant storage tank comprising a plurality of individually controllable heater elements ( 7, 8   a,  8 b ). A coolant storage tank comprising a first heater element ( 7 ) located at a base of the coolant storage tank and a second heater element ( 8   a ) is also disclosed. A coolant storage tank comprising a first coolant storage compartment ( 50 ) in fluid communication with a second coolant storage compartment ( 51 ), the first coolant storage compartment including at least a first heater element ( 54 ) and wherein the second coolant storage compartment is unheated is also disclosed. A method of melting frozen coolant in a coolant storage tank is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 16/387,393 filed Apr. 17, 2019, which is a Continuation of U.S.patent application Ser. No. 15/313,318 filed Nov. 22, 2016, which is aNational Phase entry of International Application No. PCT/GB2015/051341filed May 7, 2015, which claims priority to Great Britain ApplicationNo. 1409279.5 filed May 23, 2014, the disclosures of which areincorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

This invention relates to a coolant storage tank for a fuel cell system.In particular, it relates to a fuel cell water storage tank. Theinvention also relates to a fuel cell system including said coolantstorage tank.

Conventional electrochemical fuel cells convert fuel and oxidant intoelectrical energy and a reaction product. A common type ofelectrochemical fuel cell comprises a membrane electrode assembly (MEA),which includes a polymeric ion (proton) transfer 10 membrane between ananode and a cathode and gas diffusion structures. The fuel, such ashydrogen, and the oxidant, such as oxygen from air, are passed overrespective sides of the MEA to generate electrical energy and water asthe reaction product. A stack may be formed comprising a number of suchfuel cells arranged with separate anode and cathode fluid flow paths.Such a stack is typically in the form of a block comprising numerousindividual fuel cell plates held together by end plates at either end ofthe stack.

It is important that the polymeric ion transfer membrane remainshydrated for efficient operation. It is also important that thetemperature of the stack is controlled. Thus, coolant water may besupplied to the stack for cooling and/or hydration. Accordingly a fuelcell system may include a water/coolant storage tank for storing waterfor hydration and/or cooling of the fuel cell stack, for example. If thefuel cell system is stored or operated in sub-zero conditions, the waterin the fuel cell stack and water storage tank may freeze. The frozenwater may cause blockages that hinder the supply of coolant or hydrationwater to the fuel cell stack. This is a particular problem on shut-downof the fuel cell system when the water in the water storage tank is nolonger heated by its passage through the stack and may freezecompletely. In such an event sufficient liquid water may not beavailable for hydration and/or cooling. This may prevent the fuel cellstack from being restarted or operating at full power until the frozenwater has been thawed. It is known to provide a heater in the fuel cellsystem, which operates on stored energy, such as from a battery, andmaintains the fuel cell system at above-zero temperatures to preventfreezing occurring. The battery power is, however, limited and the fuelcell system may experience freezing if the battery fails or becomesdischarged.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the invention we provide a coolantstorage tank for storing coolant in a fuel cell system, the coolantstorage tank comprising a plurality of individually controllable heaterelements.

This is advantageous as this arrangement of heater elements has beenfound to be particularly effective for quickly and efficiently frozencoolant in the coolant storage tank.

This enables the fuel cell system, which the tank supplies with purecoolant, to be supplied with the coolant it requires quickly so that itcan operate at full power. The coolant is typically water although itmay comprise glycol or other coolant.

The coolant tank may be associated with a controller, the controllerconfigured to sequentially activate the individually controllable heaterelements. Sequential activation may comprise additionally activating theheater elements over time. It may comprise activating a first heaterelement, then subsequently activating a second heater element whiledeactivating or changing the power delivered to the first heaterelement. The controller may be configured to “sequentially activate”over time or in response to a measure of how the coolant is melting inthe tank.

At least one of the plurality of individually controllable heaterelements may comprise;

-   -   a heater element located at or in a base of the coolant storage        tank;    -   a heater element extending upwardly from a base of the coolant        storage tank;    -   a heater element extending downwardly from an upper surface of        the tank;    -   a heater element located along a side of the tank; and

a heater element extending from a side of the tank.

The coolant storage tank may comprise a first heater element located ata base of the coolant storage tank and a second heater element extendingupwardly from the base or downwardly from an upper surface. Theprovision of a base heater in combination with a heater that extendsupwardly or downwardly from the base/upper surface has been found toprovide efficient thawing of the frozen coolant.

The coolant storage tank may include two or more heater elements thatare spaced in a vertical direction, and a first of the individuallycontrollable heater elements may be disposed at the bottom of the tank.The provision of heater elements distributed vertically in the tank isadvantageous as heat is delivered at a plurality of levels in the tank.

The plurality of controllable heater elements or a subset thereof mayeach be heated using at least one of ohmic or inductive heating.Further, the plurality of controllable heater elements or a subsetthereof may comprise a local heater element which is configured togenerate heat within the tank. Alternatively, the heater elements maycomprise remote heater elements which are configured to include a heattransfer element to transfer heat generated outside the tank to thetank. For example, a heat pipe may transfer heat generated by a flow ofexhaust gas into the tank or a hydrogen catalytic heater or flammablegas burner may transfer heat to the tank via a working fluid and heatexchanger that is located in the tank. The use of the same fuel as usedin the fuel cell of which the coolant storage tank may form part isadvantageous.

The coolant storage tank may comprise a first coolant storagecompartment and a second coolant storage compartment, the second coolantstorage compartment in fluid communication with the first coolantstorage compartment, wherein the first coolant storage compartmentincludes the individually controllable heater elements and the secondcoolant storage compartment is unheated. The provision of an unheatedcompartment allows the heater's energy to be focused on a proportion ofthe coolant in the tank to improve start-up times of the fuel cellsystem.

The second coolant storage compartment may be separated from the firstcoolant storage compartment by a wall, the wall including an aperturetherein for providing fluid communication between the compartments. Thewall substantially prevents convection currents from entering the secondcompartment thereby helping to thaw the coolant in the first compartmentby limiting the amount of frozen coolant the heat from the heaterelements is applied to.

According to a further aspect of the invention we provide a fuel cellsystem comprising the coolant storage tank of the first aspect.

This is advantageous as the coolant storage tank can supply a fuel cellstack with coolant quickly in the event of freezing conditions.

A controller may be provided which is configured to activate theindividually controllable heater elements. The control circuit may beconfigured to sequentially activate the individually controllable heaterelements.

The controller may be configured to monitor a rate of fluid deliveredfrom the tank and control the plurality of individually controllableheater elements accordingly.

The fuel cell system may be configured to drive coolant in the fuel cellsystem to the coolant storage tank and/or arranged such that coolantdrains to the coolant storage tank at least on shut-down of the fuelcell system. The fuel cell may be configured to provide power to saidindividually controllable heater elements. Alternatively, or inaddition, stored energy or energy from an alternative source may be usedto power the heater elements. The controller may be configured toactivate a first heater element of the plurality of individuallycontrollable heater elements for a first period (possibly using powerfrom the fuel cell) at a first power level, provide coolant heated bysaid first heater element from the tank to said fuel cell and activateat least one of the plurality of heater elements for a second period(possibly using power from the fuel cell) at a second power level. Thesecond power level may be higher than the first power level.

According to a further aspect of the invention, we provide a method ofmelting frozen coolant in a coolant storage tank of a fuel cell system,the coolant storage tank including at least one heater element formelting frozen coolant within said tank, said coolant storage tankconfigured to supply coolant to a fuel cell of the fuel cell system forat least cooling the fuel cell during operation, the method comprisingthe steps of;

-   -   operating said fuel cell without coolant from the coolant        storage tank;    -   operating in a first melt mode comprising activating the at        least one heater element to    -   melt at least part of the frozen coolant within the coolant        storage tank;    -   supplying the melted coolant to the fuel cell; operating in a        second melt mode different to the first melt mode.

This is advantageous as the fuel cell can be operated without coolantcooling for a limited period of time in the first melt mode to liberatea small quantity of coolant from the frozen coolant tank. Once aquantity of coolant is received from the coolant tank for cooling thefuel cell stack, which may comprise less than the total amount of frozencoolant in the tank, a second melt mode can be adopted to melt a furtherquantity of the frozen coolant in the tank. The further mode may utilizedifferent heaters located at different positions in the tank. Thus, thefirst heater may melt the frozen coolant in a first zone in the tank anda second heater may melt the coolant in a second zone. By dividing thetank into zones using a plurality of individually controllable heaters,the frozen coolant in the tank is melted incrementally at differentlocalities in the tank. Thus, the first melt mode may be used to melt asmall quantity of coolant such that the fuel cell can be cooled to alevel that enables a higher power output from the fuel cell. The use ofindividually controllable heaters enables the method to apply differentmelting modes depending on the quantity of coolant that has beenliberated from the frozen coolant in the tank at a particular time. Themelt modes may comprise controlling which heaters are active, which areunactive, or the power supplied to the heaters. The fuel cell or abattery or other stored energy may be used to power the heater elementsand different power sources may be used in different modes. For example,the fuel cell may power the heaters in the first and second modes.Alternatively, a battery may be used for the first melt mode and thefuel cell for the second melt mode.

The coolant storage tank may comprise a first heater element and asecond heater element and wherein the first melt mode comprisesactivating the first heater element and the second melt mode comprisesactivating the second heater element. The second heater element may beactivated in addition to the first heater element. Alternatively, thesecond heater element may be activated and the first heater element isdeactivated (or the power supplied altered).

In the second melt mode, more power may be supplied to the heaterelement(s) from the fuel cell than in the first melt mode. Thus, as thefrozen coolant is melted, coolant is available to the fuel cell forcooling. Accordingly, the power output of the fuel cell can be increasedand supplied to the heater elements in the tank for thawing morecoolant.

The first melt mode may be operated for a first period of time and thesecond melt mode may be operated subsequently for a second period oftime. The period of time may be predetermined or it may be calculated bymonitoring parameters of the fuel cell during operation, such as fuelcell temperature, power output or exhaust composition, for example.

The melt modes may comprise a substantially continuous ramp up in powersupplied to the heater elements as power becomes available from the fuelcell.

At least one further melt mode may be provided for subsequent operationafter the first and second melt modes, the further melt mode(s)comprising;

-   -   a) activating a different heater element or combination of        heater elements than a previous melt mode; or    -   b) operating the heater element(s) at a different power level        than a previous melt mode.

The heater elements may be sized, shaped or configured to operateeffectively during each of the melt modes, which can advantageously makegood use of the power available from the fuel cell during each mode.Thus, a first heater element, which may be active during a first meltmode may be located near the outlet of the coolant tank so that thecoolant obtained by melting the frozen coolant can be easily extractedfrom the tank for supply to the fuel cell. Alternatively or in addition,the heater element active during the first melt mode may be smaller thanthe one or more other heater elements or configured to dissipate itsheat in a smaller volume of the tank than the one or more other heaterelements. Thus, the heater element may concentrate its heat output(possibly from limited output power of the fuel cell) on a small volumeof frozen coolant so that it can melt that frozen coolant and supply theresulting coolant to the fuel cell.

The fuel cell may then be able to increase its power output with thatsmall amount of coolant that is now available. Further heater elements,which may be configured such that their heat output is not asconcentrated in such a small volume, can then be activated at a higherpower level, which may now be available for melting more of the frozencoolant in the tank.

The fuel cell system as described above may be configured to operate inaccordance with the method of the above aspect. The method of operationmay be applied to any of the other aspects of the invention.

According to a third aspect of the invention, we provide a coolantstorage tank for storing coolant in a fuel cell system comprising afirst heater element located at or in a base of the coolant storage tankand a second heater element comprising at least one of;

-   -   a heater element extending upwardly from the base;    -   a heater element comprising a plurality of heater sub-elements        extending upwardly from the base;    -   a heater element extending downwardly from an upper surface of        the tank;    -   a heater element comprising a plurality of heater sub-elements        extending downwardly from an upper surface of the tank;    -   a heater element extending along a side of the tank; and a        heater element extending from    -   a side of the tank.

This is advantageous as the coolant storage tank may have two or moreactive heat elements arranged at different locations in the tank forefficiently melting frozen coolant that may form in the tank. The secondheater element may extend into the tank to effectively thaw frozencoolant that may form around it in the tank. The heater elements maycomprise elongate heaters that cantilever from the base, upper surface,a lid or side of the tank or may comprise a plurality of sub-heatersarranged in a chain. The subheaters may be individually controllable.

The first heater element may form at least part of the base of thecoolant storage tank.

The first heater element may extend over the substantially the wholebase or only a part of it. If it extends over only part of the base, itmay do so at a tank outlet region, adjacent the outlet of the tank.

The second heater element may receive electrical power through the baseof the water storage tank. The first and second heater elements may beindividually controllable.

The first heater element may comprise an electrically powered heaterelement.

The second heater element may comprise a heat pipe having an evaporatorend and a condenser end, the evaporator end arranged to receive heatfrom the first heater element.

The second heater element may comprise an elongate member. This isadvantageous as the heater element can extend into the mass of frozencoolant should the coolant in the tank freeze.

Further heater elements may be provided. Further heater elements in thebase may be provided and/or further heater elements that extend upwardlyfrom the base may be provided and/or further heater elements that extenddownwardly from the upper surface.

The coolant storage tank may include a third heater element, located atleast partly above or below the second heater element, the third heaterelement being individually controllable.

Optionally, the second heater element comprises a proximal end locatedat the base or upper surface of the tank and a distal end opposite theproximal end, the third heater element arranged to extend from thedistal end of the second heater element. A plurality of third heaterelements may be provided.

A controller may be configured to activate the heater elements based onthe level of coolant in the coolant storage tank. A controller may beconfigured to maintain coolant within the coolant storage tank for aperiod after activation of the heater elements. The period may comprisea predetermined amount of time. The period may be determined using ameasure of the energy supplied to the first and/or second heaterelements. This is advantageous as supplying more energy than required tojust melt the coolant, reduces the risk of it refreezing when it leavesthe water tank.

The coolant storage tank may comprise a first coolant storagecompartment and a second coolant storage compartment, the second coolantstorage compartment in fluid communication with the first coolantstorage compartment, wherein the first coolant storage compartmentincludes the first and second heater elements and the second coolantstorage compartment is unheated.

The second coolant storage compartment may be separated from the firstcoolant storage compartment by a wall, the wall including an aperturetherein for providing fluid communication between the compartments.

According to a fourth aspect of the invention we provide a fuel cellsystem comprising the coolant storage tank of the third aspect of theinvention.

According to a fifth aspect of the invention we provide a coolantstorage tank for storing coolant in a fuel cell system, the coolantstorage tank comprises a first coolant storage compartment and a secondcoolant storage compartment, the second coolant storage compartment influid communication with the first coolant storage compartment, thefirst coolant storage compartment including at least a first heaterelement and wherein the second coolant storage compartment is unheated.

The second coolant storage compartment may be separated from the firstcoolant storage compartment by a partition wall, the wall including anaperture therein for providing fluid communication between thecompartments.

The first heater element may form at least part of the base of the firstcoolant storage compartment.

The coolant storage tank may include a plurality of individuallycontrollable heater elements.

The first coolant storage compartment may include a first heater elementlocated at a base of the coolant storage compartment and a second heaterelement extending upwardly from the base. Alternatively, the secondheater element may extend downwardly from an upper surface of the tank.The second heater element may comprise an electrically powered heaterelement. The second heater element may comprise a heat pipe having anevaporator end and a condenser end, the evaporator end arranged toreceive heat from the first heater element. The second heater elementmay comprise an elongate member. A plurality of second heater elementsmay be provided.

The coolant storage tank may include a third heater element, located atleast partly above the second heater element or elements, the thirdheater element being individually controllable. A plurality of thirdheater elements may be provided.

A controller may be configured to activate the heater elements based onthe level of coolant in the coolant storage tank. A controller may beconfigured to maintain coolant within the coolant storage tank for aperiod after activation of the first heater element wherein the periodcomprises;

-   -   a) a predetermined amount of time; or    -   b) the period is determined using a measure of the energy        supplied to the first and/or second heater elements.

The first coolant storage compartment may include a plurality ofindividually controllable heater elements.

According to a sixth aspect of the invention, we provide a fuel cellsystem comprising the coolant storage tank of the fifth aspect of theinvention.

DESCRIPTION OF THE DRAWINGS

There now follows, by way of example only, a detailed description ofembodiments of the invention with reference to the following figures, inwhich:

FIG. 1 shows an example coolant storage tank of a fuel cell system;

FIG. 2 shows a diagrammatic view of a first example coolant storage tankincluding heater elements;

FIG. 3 shows a diagrammatic view of a second example of the heaterelements in the coolant storage tank;

FIG. 4 shows a diagrammatic view of a third example of the heaterelements in the coolant storage tank;

FIG. 5 shows a diagrammatic view of a fourth example coolant storagetank divided into compartments;

FIG. 6 shows a fifth example coolant storage tank having heater elementsextending downwards;

FIG. 7 shows a sixth example coolant storage tank having inclined sidewalls and heater element associated with the side walls; and FIG. 8shows a seventh example coolant storage tank;

FIG. 9 shows an eighth example coolant storage tank;

FIG. 10 shows a plan view of the tank shown in FIG. 9;

FIG. 11 shows a ninth example coolant storage tank;

FIG. 12 shows a tenth example coolant storage tank.

DETAILED DESCRIPTION OF THE DRAWINGS

The figures show a coolant storage tank 1 of a fuel cell system 2. Inthis example, the coolant comprises water, although it will beappreciated that other coolants could be used. Further, it will beappreciated that the storage tank of the invention may store liquidsother than coolant and has application in liquid storage tanks thatexperience freezing conditions or in which the liquid requires heating.The coolant or water storage tank 1 stores pure water for the hydrationand evaporative cooling of a fuel cell stack 3.

The water storage tank 1 comprises a hollow body 4 having an inlet 5 forreceiving water, which may be extracted from an exhaust fluid flow ofthe fuel cell stack 3, and an outlet 6 for supplying water for use bythe fuel cell stack 3. The water storage tank 1 includes a plurality ofindividually controllable heater elements 7, 8 a, 8 b (shown in the moredetailed view of FIG. 2).

The water storage tank 1 further includes a gauge 10 for displaying to auser how much water is present in the water storage tank 1. Thestructure of such a gauge will be known to those skilled in the art andwill only be described briefly here. The gauge 10 comprises a hollowcolumn 12 containing a float 11. A flow path 13 connects a base 14 ofthe column 12 to a base 15 of the water storage tank 1 such that theyare in fluid communication. Water in the water storage tank 1 is able toenter the column 12 via the flow path 13 and act on the float 11 to showthe water level. As an alternative or in addition to the gauge 10, alevel sensor, such as a capacitive sensor may be present in the tank orcolumn 12. The level sensor may include a display for displaying itsoutput.

The sensor may report the level to a controller, which may use theinformation to sequentially activate heaters and/or choose which heatersto activate. A drain 16 is present in the flow path 13 for drainingwater from the column 12. A valve 17 is also provided in the flow path13 between the water storage tank 1 and the drain 16 to control the flowof water from the tank 1 to the column 12.

The outlet 6 includes a valve 18 for controlling the flow of water fromthe tank 1 out of the outlet 6 into an outlet conduit 19, whichtransports water to the remainder of the fuel cell system. A drain 20 isalso provided at the outlet 6 downstream of the valve 18 for drainingthe outlet conduit 19. The outlet conduit 19 includes a pump 21 forpumping water from the tank along the outlet conduit 19 to the fuel cellstack. The body 4 includes a further drain 22 in the base 15 of the tank1 and an air inlet 23.

FIG. 2 shows the base 15 of the water storage tank 1. A first heaterelement comprising a base heater element 7, is located within the base15 of the body 6. The base heater element 7 may comprise a plate heaterthat extends over the base of the body 4. The base heater element 7 isan electrically powered heater element. A second heater element,comprising an upward heater element 8 a, extends substantially upwardsfrom the base 15. The upward heater element 8 a comprises an elongatemember that extends into the hollow interior of the water storage tank1. The upward heater 8 a is an electrically powered heater element andreceives its electrical power through the base 15. The upward heaterelement 8 a is separately controllable from the base heater element 7.In this embodiment, a second upward heater element 8 b is provided(which may be considered a third heater element). The second upwardheater element 8 b takes the same form as the first upward heaterelement but extends from a different part of the base 15. The secondupward heater element 8 b can be controlled separately from the base andfirst upward heater elements 7, 8 a.

FIG. 1 shows the water storage tank 1 on shut down of the fuel cellsystem 2. The system 2 may be configured and arranged such that liquidwater 24 in the system 2 will drain to the water storage tank 1.Alternatively or in addition, the fuel cell system 2 may actively drivewater to the water storage tank at least on shut down of the fuel cellsystem. Thus a purge gas may be used to flush any liquid water out ofthe fuel cell stack 3 for example and into the water storage tank 1.

The valve 17 and the valve 18 are actuated to prevent water in the waterstorage tank 1 leaving the tank 1 via the flow path 13 and outlet 6. Thedrains 16 and 20 are opened to drain the column 12 and the outletconduit 19 of any water that may be present therein.

This ensures the gauge 10, outlet 6 and outlet conduit 19 remain clearof any ice that may form.

In the event of freezing conditions, the water 24 in the tank 1 isallowed to freeze. The system 2 may not include an auxiliary heater tomaintain an above-freezing temperature while the system 2 is powereddown. On restarting the system 1, water may be required 15 for coolingthe fuel cell stack 3 and/or hydration of the fuel cell membranes. Thus,if the water in the tank 1 is frozen, it must be thawed quickly so thatit is available to the stack 3.

The stack 3 may be started at a reduced power level, such as 10% ofnormal power, without injecting any water for evaporative cooling fromthe water tank. The oxidant may be driven through the stack such that itoperates at high stoichiometry in order to keep the stack cool untilwater from the tank 1 is available for evaporative cooling. Theelectrical power generated by the stack 3 is, in this example, used topower the heater elements 7, 8 a, 8 b. This is advantageous as the powerrequired to melt the ice is generated by the stack itself rather than bydraining a battery. It is known that batteries may experience lowperformance in cold temperatures and therefore using the stack power,possibly operating in a low power mode, is beneficial. The heaterelements are individually controllable and therefore may be activated asrequired. In this example, a controller 25 activates all of the heaterelements; the base heater element 7 and the two upward heater element 8a and 8 b. The heater elements will melt the ice in the tank.

The controller 25 may retain valve 18 in the closed position for aheating period after the heater elements 7, 8 a, 8 b have begun to meltthe ice. This will retain water around the heater elements to act as aconductor/convection medium to assist in melting the remainder of theice in the tank 1. The heating period may comprise a predeterminedperiod of time. Alternatively, the heating period may be determinedbased on the amount of ice that has been melted, which may be determinedby measuring the current draw of the heater elements, using atemperature sensor in the tank 1 or any other suitable means.

The valve 18 may then be opened (with drain 20 now closed) to allowwater to be supplied to the fuel cell stack 3. The valve 17 may also beopened (with drain 16 now closed) to activate the gauge 10. As waterbecomes available from the tank 1, the power output of the stack 3 maybe increased. Accordingly, more power may be available to the heaterelements to thaw any remaining ice. Alternatively, the additional powermay be provided for other uses.

As the melted water leaves the tank 1 for use in the system 2, thecontroller 25 may deactivate the upward heaters 8 a and 8 b. Forexample, if the water/ice level in the tank 1 is below that of theupward heaters 8 a, 8 b, then they will not effectively heat the ice.Thus, it is advantageous for the controller 25 to deactivate the upwardheater elements 8 a, 8 b. Alternatively, the controller 25 maydeactivate all of the heater elements 7, 8 a, 8 b as soon as sufficientwater is available for the stack, whether or not there is still icepresent in the tank 1. Accordingly, the system 2 may rely on water,which has been heated by its passage through the stack, entering orre-entering the tank 1 to thaw any remaining ice. Alternatively, thecontroller 25 may wait for all the ice to have melted beforedeactivating all of the heater elements 7, 8 a, 8 b. The combination ofa base heater and at least one upward heater has been found to providean efficient way of thawing ice in the water tank. The provision ofindividually controllable heater elements is also advantageous because,for example, the heater elements that can effectively melt the ice canbe activated in preference to others.

A further method of operation may comprise a first melt mode and asecond melt mode operated sequentially. The melt modes may differ withrespect to the heater element that is active, the combination of heaterelements that are active, the power supplied to the or each heaterelement or a combination of the above. As an example, the method ofmelting ice in the water storage tank of a fuel cell system may compriseoperating the fuel cell stack 3 without evaporative cooling using waterfrom the tank 1 at a reduced power. The reduced power generated by thestack 3 may be supplied to the heater elements operating in the firstmelt mode. In the first melt mode, in this example, one of the heaterelements 8 b is supplied with power while the other heater element arenot active. It has been found that the fuel cell stack can be run for alimited period of time without evaporative cooling. By focusing thepower generated during this limited period of time into one of theheater elements, a proportion of the ice may be thawed. This water maythen be supplied to the fuel cell stack for evaporative cooling of thestack.

With this limited quantity of water available it has been found that thefuel cell stack can be operated at a higher power. Thus, a different,second melt mode can be used to melt a further quantity of water.

The second melt mode may comprise using the additional power obtainedfrom the fuel cell stack 3 to activate a further one of the plurality ofheater elements, such as heater element 8 a. Thus, in the second meltmode, the heater element 8 a may be activated to melt the ice around it.In the first melt mode, the heater element 8 b is used to melt the icearound it for supplying to the fuel cell stack 3. In the second meltmode, a different quantity of ice, at a different location in the tank,that which surrounds heater element 8 b is melted. Alternatively bothheater elements 8 a and 8 b may be activated.

The use of individually controllable heaters allows a controller toactivate the heaters sequentially as power from the fuel cell stackbecomes available. Further, as more water is melted from the water tank,the fuel cell stack can be operated at higher powers.

Thus, the provision of different melt modes applied sequentially canadvantageously use the power that is available. The position of theheater elements in the tank conceptually breaks the tank into zones suchthat sections of an ice mass in the tank can be incrementally melted.

The melt modes may be applied for predetermined periods of time. Forexample, the first melt mode may be operated for 30 seconds beforemoving to the second melt mode.

Alternatively, the duration of each melt mode may be determined bymonitoring the performance of the heater elements and/or the fuel cellstack. For example, the controller may measure the quantity of waterthat is melted during a particular mode and may move to the next modewhen a predetermined quantity of water is received from the tank.Alternatively or in addition, the controller may measure the temperatureof the fuel cell stack 3, and determine when the transition betweenmodes should occur.

Further melt modes may be provided and may comprise activating adifferent heater element or combination of heater elements than aprevious melt mode; or operating the heater element(s) at a differentpower level, supplied from the fuel cell to the heater elements, than aprevious melt mode.

The heater element configured to be operated during the first melt modemay have a different configuration and/or location to the other heaterelements. For example, it may be located closest to a water outlet ofthe water tank. Alternatively or in addition, its energy output per unitvolume it occupies may be higher than other heater elements in the tank.This is advantageous as in the first melt mode, limited power may beavailable but melting a small quantity of water quickly has been foundto enable the power output of the fuel cell to be increased, therebyaiding further melting of ice. Providing a compact heater element foractivation during the first melt mode may enable effective and rapidmelting of a small quantity of ice during the first melt mode.

FIG. 3 shows an alternative arrangement of the heater elements in thetank 1. In this example, an electrically powered base heater element 30is provided as in the previous example. Three second heater elementscomprising upward heater elements 31 a, 31 b, 31 c are provided.However, in this example, the upward heater elements 31 a-c compriseheat pipes rather than electrically powered heater elements. The heatpipes 31 a-c are arranged to transfer thermal energy from the baseheater element 30 into the mass of ice that may form in the tank 1. Theheat pipes 31 a-c each comprise an evaporating end 32 a-c and acondensing end 33 a-c. The evaporating ends 32 a-c are located incontact with the base of the tank 1 and therefore receive thermal energyfrom the base heater element 30. A working fluid present in each heatpipe absorbs thermal energy at the evaporating end 32 a-c andevaporates. The working fluid is transported along the pipe to thecondensing end 33 a-c where the thermal energy is transferred to theice/water in the tank 1 and the working fluid condenses. The workingfluid is then transported to the evaporating end 32 a, 32 b, possibly bywicking, to continue the cycle.

The heat pipes 31 a-c may be of different heights and/or locations andmay therefore extend to different points within the tank 1. For example,there may be a higher concentration of heater elements 30, 31 a-c aroundthe outlet 6. This would be advantageous as the ice would be melted morerapidly around the outlet 6 so that water is quickly available to flowfrom the outlet 6. This higher concentration of heater elements aroundthe outlet can be applied to any heater element type described herein.

The operation of this embodiment is similar to the previous embodiment.However, the controller 35 only controls the single base heater 30 asthe heat pipes 31 a-c passively transfer thermal energy from the baseheater element 30 to the interior of the tank 1. In a modification toFIG. 3, the base heater element 30 may be divided into severalindependently controllable sections. The sections may be arranged in anylayout over the base 15, for example they may be concentric or dividethe base into sectors. The sections may extend over a part of the baseassociated with one or more of the heat pipes. It will be appreciatedthat the division of the base heater into a plurality of individuallycontrollable base heater sections can be applied to a water tank 1 withor without any second heater elements and whether or not they compriseheat pipes.

FIG. 4 shows an alternative arrangement of the heater elements in thetank 1. In FIG. 4, a first heater element comprising base heater element40 is provided. Two second heater elements comprising upward heaterelements 41 a and 41 b are also provided.

The upward heater elements are electrically powered and receive theelectrical power through the base 15. In this embodiment, third heaterelements 42 a, 42 b are provided, comprising upper heater elements thatextend higher in the tank 1 than the second heater elements 41 a, 41 b.The third heater elements 42 a, 42 b are electrically powered heaterelements and have an elongate form. The upward or second heater elements41 a, 41 b comprise a proximal end 43 a, 43 b that extends from the baseand a distal end 44 a, 44 b opposite the proximal end. The third heaterelements 42 a, 42 b, in 15 this example, extend from the distal end 44a, 44 b of the upward heater elements 41 a, 41 b. The third heaterelements 42 a and 42 b are controlled and receive electrical powerthrough a connection 46 a, 46 b between the second and third heaterelements.

In use, the controller 45 can individually control the base heaterelement 40, first upward heater element 41 a, second upward heaterelement 41 b, first upper heater element 42 a and second upper heaterelement 42 b. The controller 45 may initially activate all of the heaterelements when ice is detected in the water tank 1. As in the firstexample, energy to power the heater elements may be obtained byoperating the fuel cell stack in a reduced power mode until water fromthe tank 1 is available. Alternatively a further energy source may beused. As the ice melts and the ice/water level in the tank 1 falls, thecontroller may sequentially deactivate the heater elements. Inparticular, the controller 45 may first deactivate the upper heaterelements 42 a, 42 b. As the ice/water levels drops further, thecontroller may deactivate the upward heater elements 41 a, 41 b andthen, when sufficient water is available or when all of the ice hasmelted, for example, the controller may deactivate the base heaterelement 40.

FIG. 5 shows a further example of a water storage tank 1. In thisembodiment, the water storage tank 1 is divided into two compartments; afirst compartment 50 and a second compartment 51. The second compartment51 is separated from the first compartment 50 by a partition wall 52.The portion wall 52 includes an aperture 53 which provides fluidcommunication between the first and second compartments 50, 51.

The first compartment 50 includes a first heater element 54, a secondheater element 55 and a third heater element 56 comprising a base heaterelement, upward heater element extending from the base 15 and an upperheater element respectively. The heater elements thus have a similarconstruction to that shown in FIG. 4 except that only one upward 55 andone upper heater element 56 is provided. The second compartment 51 doesnot contain any heater elements and is thus unheated. The secondcompartment may be located to the side of the first compartment and maybe located such that it is not wholly directly above the heaterelements. Further, the majority of the second compartment may be offsetfrom an area extending above the heater elements. The partition wall mayinsulate the first compartment from the second compartment. Thisarrangement allows the heater elements to act primarily or wholly on thevolume of ice/water in the first compartment 50.

The first compartment 50 is sized to hold a particular quantity ofwater, which may be the minimum amount of water required for the fuelcell system 2 to operate at full power or a particular power requirementless than full power (such as a power level of 30, 40, 50, 60, 70, 80 or90 percent of full operating power). The second compartment 51 may holdadditional water or the extra water required such that the fuel cellsystem can operate at full power.

The arrangement of the compartments is advantageous as the water storagetank compartmentalizes the ice so that the heater elements can thaw adesired quantity (determined by the compartment sizes) of the ice foroperation. This improves the startup time of the fuel cell systemachieving a desired output power.

In use, as in the first example, water in the system 2 may be driven, onshut-down, to the water storage tank 1 and may be received in either orboth compartments 50, 51. The outlet 6 is located in the firstcompartment (as is the valve 17) and the aperture 53 allows water in thesecond compartment 51 to flow to the first compartment and leave thetank 1 by the outlet 6. The water is allowed to freeze in bothcompartments 50, 51.

On start-up of the fuel cell system 2, the heater elements 54, 55, 56are activated which heats the ice in the first compartment 50. Theoffset location of the second compartment and the presence of thepartition wall 52 allows the heater element 54, 55, 56 to thaw the icein the first compartment without substantially acting on the ice in thesecond compartment 51. Any convection currents caused by the heaterelements that may aid thawing do not substantially enter the secondcompartment 51 due to the partition wall.

Thus, the configuration of the tank promotes the thawing of ice in thefirst compartment 50 over the second compartment 51. FIG. 5 shows theice/water level 57 in the first compartment 50 and the ice/water level58 in the second compartment 51.

The ice/water level 57 is lower than the ice/water level 58. FIG. 5 thusshows the water tank 1 after the heater elements 54, 55, 56 have beenactive for some time and have started to melt the ice in the firstcompartment 50.

The upper heater element 56 may be deactivated as the level 57 fallsbelow it, followed by the upward heater element 55 and the base heaterelement 54. The ice in the second compartment 51 may remain frozen afterthe first compartment 50 has reached a minimum level and is supplyingthe required quantity of water to the fuel cell system 2.

The water in the system 2 may be cycled through the water storage tank 1and therefore the water entering the tank 1 may act to thaw the ice inthe second compartment 51.

The system may be configured such that the ice in the second compartment51 will thaw over time due to the increased system temperatureassociated with operation of the fuel cell system.

Thus, even after the heater elements 54, 55, 56 have been deactivated,the water in the second compartment may thaw and enter the firstcompartment 50 through the aperture 53 to augment the water from thefirst compartment.

Given that the heater elements are concentrated on thawing only theminimum amount of ice required (due to its compartmentalization in thewater tank), the fuel cell system can operate efficiently and start-upquickly.

FIG. 6 shows a further embodiment of an alternative arrangement of theheater elements in the tank 1. In FIG. 6, a first heater elementcomprising base heater element 60 is provided. A second heater element61 and a third heater element 62 are also provided. The second and thirdheater elements both extend from an upper surface 63 of the tankdownwardly into the hollow interior of the tank 1. The second and thirdheater elements are individually controllable. Further, the second andthird heater elements are electrically powered and receive theirelectrical power through the upper surface 63.

The downwardly extending second and third heater element 61, 62 arelocated at different positions in the volume of the tank interior andcan therefore be considered to be associated with a “zone” in the tankwhere that specific heater element is used to melt ice present in thatzone. A controller 65 may be configured to activate the heaters asrequired, which may be over a plurality of melt modes during which theactive heaters change and/or different amounts of power are supplied tothe heater elements.

FIG. 7 shows a further embodiment in which the tank 1 includes slopingside walls 71.

The or each side wall of the tank is inclined such that the distancebetween the side walls decreases towards a base 72 of the tank 1. Inthis embodiment, a base heater element is not provided. A first heaterelement comprises a side wall heater 73 that extends within the body 4of the tank 1 along the side wall. A second heater element comprises afurther side wall heater 74 on an opposite side. Third and fourth heaterelements 76, 77 are provided that extend downwardly from an uppersurface (not shown) of the tank 1.

The first, second, third and fourth heater elements are individuallycontrollable and are controlled by controller 75. The controller 75 canimplement any appropriate scheme for melting ice that may form withinthe tank 1. For example, it may be configured to sequentially activateone or more of the heater elements over a plurality of melt modes.

FIG. 8 shows a further embodiment in which a first heater element 80 ispresent in a base of the tank and a second heater element 81 extendsdownwardly from an upper surface 83 of the tank. The upper surface ofthe tank from which the heater element depend may comprise a lid forgaining access to the tank. This is advantageous as the lid providesconvenient access to the heater elements for servicing and the like.This configuration may be applicable to any of the above embodiments.

The second heater element comprises an elongate frame 82 which carries aplurality of sub-heater element 81 a, 81 b and 81 c located at spacedlocations along the frame. The sub-elements are individuallycontrollable. The sub-heater elements, like the individuallycontrollable heater elements described above, provide a further meansfor concentrating power from the fuel cell or other power source atparticular locations within the tank 1.

FIGS. 9 and 10 show a further example having individually controllableheaters. The tank 91 includes a base heater element 92 and a pluralityof inductive heater elements 93 a-c. The inductive heater elementscomprise induction coils that surround the tank and generate anelectromagnetic field on the application of an alternating current.Thus, the coils of the inductive heater elements may be supplied by DCto AC converter, which receives a DC current from the fuel cell stack.The inductive heater elements are configured to act, by virtue of theelectromagnetic field, on a conductive assembly, which in this examplecomprises a metal tube assembly 94 that extends within the tank.

Although the inductive heater elements 93 a-c are configured to act on acommon conductive assembly, they are arranged at different (vertical)locations around the tank and therefore preferentially heat a localizedportion of the conductive assembly. It will be appreciated that aplurality of conductive assemblies may be provided to each receive theinductive energy for the inductive heater elements or a subset thereof.Nevertheless, while a particular inductive heater element may primarilyheat its associated conductive assembly, it may also induce a current inthe other conductive assemblies associated with other inductive heaterelements to a lesser degree. The metal tube assembly 94 comprises twoconcentric tubes of different diameter. The tubes may be perforated orhave apertures therein to allow any melted coolant to circulatetherearound. Thus, a controller 95 may individually control the baseheater 92 and inductive heater elements 93 a-c.

FIG. 11 shows a further example similar to the example shown in FIG. 9.In this example, five individually controllable inductive heaterelements 1103 a-e are provided at spaced locations around the tank 1101.The inductive heater elements 1103 a-e are configured to induce heatingin a conductive assembly 1104, which in this example comprises astirrer. The stirrer 1104 is configured to rotate in the tank 1101 and amotor 1105 provides the motive force. A base heater is not provided inthis example, but could be included. The stirrer 1104 comprises arotatable shaft 1106 and a plurality of arms 1107 extending therefrom.The stirrer 1105 may be advantageous in that it can move melted orpartially melted ice around the tank to encourage heat transfer throughthe tank 1101.

FIG. 12 shows a further example similar to that shown in FIG. 6. FIG. 12shows a first heater element comprising a heat exchanger 120. The heatexchanger 120 is located at the base of the tank 1. A second heaterelement 121 and a third heater element 122 are also provided. The secondand third heater elements 121, 122 both extend from an upper surface 123of the tank downwardly into the hollow interior of the tank 1. Thefirst, second and third heater elements are individually controllable.

The heat exchanger 120 comprises a remote heater as it is supplied withheat from a heat source outside the tank 1 rather than generating heatin situ. The heat exchanger 120 is supplied with heat from a hydrogencatalytic heater 124. The hydrogen catalytic heater uses a catalyst tooxidize a hydrogen fuel to generate heat. The hydrogen catalytic heater124 transfers the heat generated to a working fluid, which is circulatedvia conduits 125 to the heat exchanger 120. The heat exchanger 120 thustransfers the heat to melt any ice within the base of the tank 1. Theuse of a hydrogen catalytic heater is advantageous as a source ofhydrogen for the fuel cell is readily available. The second and thirdheater elements 121, 122 are electrically powered.

The first, second and third heater elements 121, 122 are located atdifferent positions in the volume of the tank interior and can thereforebe considered to be associated with a “zone” in the tank where thatspecific heater element is used to melt ice present in that zone. Acontroller 126 may be configured to activate the heaters 124/120, 121,122 as required, which may be over a plurality of melt modes duringwhich the active heaters change and/or different amounts of power aresupplied to the heater elements, as described in relation to previousembodiments.

It will be appreciated that the heater elements discussed in theexamples above may be of other types or a combination of types. Forexample, the heater elements may be of a type that generates heat in apart that extends within the tank, such as an electrical resistanceheater, an inductive heater or a heater element containing a phasechange material which generates heat by a reversible exothermic chemicalreaction. The heater elements may also comprise elements that receiveheat from a remote heat generation location to the tank. For example,the tank may include a heat exchanger that receives a heated heattransfer fluid, such as a glycol/water mix (the heat transfer fluidhaving a freezing temperature lower than the coolant in the tank). Aheat pipe may be used to direct heat into the tank. The heat pipe orpipes may connect to a heat transfer device, such as a heat exchanger,in the tank. Electrical resistance heaters or inductive heaters may bearranged to heat a part of a heat transfer element outside the tank, theheat transfer element configured to transfer the heat to the tank. Theheat transfer element may comprise a heat pipe or other thermallyconducting member. The heat or energy for heat generation may besupplied by an air-cooled fuel cell, a battery, a catalytic ornoncatalytic hydrogen burner, a flammable gas burner, an exhaust gasheat exchanger which may use gases exhausted from the fuel cell stack, acapacitor or supercapacitor and/or a heat storage battery, such asSchatz device.

It will be appreciated that further heater elements may be provided, ofthe base, upward or upper type, extending downward from an upper surfaceof the tank or from the sides or further heater elements arranged higheror at other positions in the tank 1. Further, combinations of the aboveheater elements may be provided in the tank 1. The heater elements mayinclude fins, bristles, spokes or other passive heating elements todistribute the thermal energy throughout the ice mass in the water tank.The heater elements may have different shapes or orientations to thosedescribed. For example, the third heater elements may be inclined to theupward direction of the second heater elements. The controller may heatthe water in the tank when it is not frozen.

What is claimed is:
 1. A method of heating coolant in a coolant storagetank to melt frozen coolant in a fuel cell system, the methodcomprising: sequentially activating at least two individuallycontrollable heater elements; wherein a controller is configured to;activate a first heater element for a first period at a first powerlevel, and activate at least one other heater elements for a secondperiod at a second power level; wherein the energy output per unitvolume occupied by the first heater element is higher than the energyoutput per unit volume occupied by the second heater element; and,wherein coolant made available from the heating is supplied to a fuelcell.
 2. The method of claim 1, in which at least one of the at leasttwo individually controllable heater elements comprises: a heaterelement located at a base of the coolant storage tank; a heater elementextending upwardly from a base of the coolant storage tank; a heaterelement extending downwardly from an upper surface of the tank; a heaterelement located along a side of the tank; or a heater element extendingfrom a side of the tank.
 3. The method of claim 1 in which the heatingelements are at least one of ohmic or inductive.
 4. The method of claim1 in which the coolant storage tank comprises a first coolant storagecompartment and a second coolant storage compartment, the second coolantstorage compartment in fluid communication with the first coolantstorage compartment, wherein the first coolant storage compartmentincludes the individually controllable heater elements and the secondcoolant storage compartment is unheated.
 5. The method of claim 4, inwhich the second coolant storage compartment is separated from the firstcoolant storage compartment by a wall, the wall including an aperturetherein for providing fluid communication between the compartments. 6.The method of claim 1, wherein the controller is configured to monitorfluid delivered from the tank and control the plurality of individuallycontrollable heater elements accordingly.
 7. The method of claim 1wherein: the controller configured to sequentially activate theindividually controllable heater elements of the coolant storage tank,and wherein the fuel cell system is configured to drive coolant in thefuel cell system to the coolant storage tank or arranged such thatcoolant drains to the coolant storage tank at least on shut-down of thefuel cell system.
 8. The method of claim 1 wherein the controller isconfigured to determine a heating period based on the current draw of atleast one heater elements and the temperature as measured by a sensor inthe tank.
 9. A method of melting frozen coolant in a coolant storagetank of a fuel cell system, the coolant storage tank including at leastone heater element for melting frozen coolant within said tank, the atleast one heater element configured to be powered by the fuel cell, themethod comprising the steps of, operating a fuel cell without coolantfrom the coolant storage tank; operating in a first melt mode comprisingactivating the at least one heater element to melt at least part of thefrozen coolant within the coolant storage tank; supplying melted coolantto the fuel cell; operating in a second melt mode different to the firstmelt mode; in the second melt mode a second heater element is activated;and more power is supplied to the second heater element from the fuelcell in the second melt mode than the power supplied to the first heaterelement from the fuel cell in the first melt mode.
 10. The method ofclaim 9, in which the fuel cell is operated at a reduce power level inthe without coolant state.
 11. The method of claim 9, in which the firstmelt mode is operated for a first period of time and the second meltmode is operated subsequently for a second period of time.
 12. Themethod of claim 9, wherein the coolant storage tank comprises a firstcoolant storage compartment and a second coolant storage compartment,wherein the second coolant storage compartment is in fluid communicationwith the first coolant storage compartment, and wherein the first andsecond heater elements are located in the same compartment.
 13. Themethod of claim 12, in which the second coolant storage compartment isseparated from the first coolant storage compartment by a partition wallincluding at last one aperture.